Hybrid coordination function (hcf) access through tiered contention and overlapped wireless cell mitigation

ABSTRACT

A method and system reduce interference between overlapping first and second wireless LAN cells in a medium. Each cell includes a respective plurality of member stations and there is at least one overlapped station occupying both cells. An inter-cell contention-free period value is assigned to a first access point station in the first cell, associated with an accessing order in the medium for member stations in the first and second cells. The access point transmits a beacon packet containing the inter-cell contention-free period value, which is intercepted at the overlapped station. The overlapped station forwards the inter-cell contention-free period value to member stations in the second cell. A second access point in the second cell can then delay transmissions by member stations in the second cell until after the inter-cell contention-free period expires. The beacon packet sent by the first access point station also includes an intra-cell contention-free period value, which causes the member stations in the first cell to delay accessing the medium until polled by the first access point. After the expiration of the intra-cell contention-free period, member stations in the first cell may contend for the medium based on the quality of service (QoS) data they are to transmit, using the Tiered Contention Multiple Access (TCMA) protocol.

BACKGROUND OF THE INVENTION

Wireless Local Area Networks (WLANS)

Wireless local area networks (WLANs) generally operate at peak speeds ofbetween 10 to 100 Mbps and have a typical range of 100 meters. Singlecell Wireless LANs, are suitable for small single-floor offices orstores. A station in a wireless LAN can be a personal computer, a barcode scanner, or other mobile or stationary device that uses a wirelessnetwork interface card (NIC) to make the connection over the RF link toother stations in the network. The single-cell wireless LAN providesconnectivity within radio range between wireless stations. An accesspoint allows connections via the backbone network, to wirednetwork-based resources, such as servers. A single cell wireless LAN cantypically support up to 25 users and still keep network access delays atan acceptable level. Multiple cell wireless LANs provide greater rangethan does a single cell, by means of a set of access points and a wirednetwork backbone to interconnect a plurality of single cell LANs.Multiple cell wireless LANs can cover larger multiple-floor buildings. Amobile laptop computer or data collector with a wireless networkinterface card (NIC) can roam within the coverage area while maintaininga live connection to the backbone network.

Wireless LAN specifications and standards include the IEEE 802.11Wireless LAN Standard and the HIPERLAN Type 1 and Type 2 Standards. TheIEEE 802.11 Wireless LAN Standard is published in three parts as IEEE802.11-1999; IEEE 802.11a-1999; and IEEE 802.11b-1999, which areavailable from the IEEE, Inc. web sitehttp://grouper.ieee.org/groups/802/11. An overview of the HIPERLAN Type1 principles of operation is provided in the publication HIPERLAN Type 1Standard, ETSI ETS 300 652, WA2 December 1997. An overview of theHIPERLAN Type 2 principles of operation is provided in the BroadbandRadio Access Networks (BRAN), HIPERLAN Type 2; System Overview. ETSI TR101 683 VI.I.1 (2000-02) and a more detailed specification of itsnetwork architecture is described in HIPERLAN Type 2, Data Link Control(DLC) Layer; Part 4. Extension for Home Environment, ETSI TS 101 761-4V1.2.1 (2000-12). A subset of wireless LANs is Wireless Personal AreaNetworks (PANs), of which the Bluetooth Standard is the best known. TheBluetooth Special Interest Group, Specification Of The Bluetooth System,Version 1.1, Feb. 22, 2001, describes the principles of Bluetooth deviceoperation and communication protocols.

The IEEE 802.11 Wireless LAN Standard defines at least two differentphysical (PHY) specifications and one common medium access control (MAC)specification. The IEEE 802.11 (a) Standard is designed to operate inunlicensed portions of the radio spectrum, usually either in the 2.4 GHzIndustrial, Scientific, and Medical (ISM) band or the 5 GHzUnlicensed-National Information Infrastructure (U-NII) band. It usesorthogonal frequency division multiplexing (OFDM) to deliver up to 54Mbps data rates. The IEEE 802.11(b) Standard is designed for the 2.4 GHzISM band and uses direct sequence spread spectrum (DSSS) to deliver upto 11 Mbps data rates. The IEEE 802.11 Wireless LAN Standard describestwo major components, the mobile station and the fixed access point(AP). IEEE 802.11 networks can also have an independent configurationwhere the mobile stations communicate directly with one another, withoutsupport from a fixed access point.

A single cell wireless LAN using the IEEE 802.11 Wireless LAN Standardis an Independent Basic Service Set (IBSS) network. An IBSS has anoptional backbone network and consists of at least two wirelessstations. A multiple cell wireless LAN using the IEEE 802.11 WirelessLAN Standard is an Extended Service Set (ESS) network. An ESS satisfiesthe needs of large coverage networks of arbitrary size and complexity.

Each wireless station and access point in an IEEE 802.11 wireless LANimplements the MAC layer service, which provides the capability forwireless stations to exchange MAC frames. The MAC frame transmitsmanagement, control, or data between wireless stations and accesspoints. After a station forms the applicable MAC frame, the frame's bitsare passed to the Physical Layer for transmission.

Before transmitting a frame, the MAC layer must first gain access to thenetwork. Three interframe space (IFS) intervals defer an IEEE 802.11station's access to the medium and provide various levels of priority.Each interval defines the duration between the end of the last symbol ofthe previous frame, to the beginning of the first symbol of the nextframe. The Short Interframe Space (SIFS) provides the highest prioritylevel by allowing some frames to access the medium before others, suchas an Acknowledgement (ACK) frame, a Clear to Send (CTS) frame, or asubsequent fragment burst of a previous data frame. These frames requireexpedited access to the network to minimize frame retransmissions.

The Priority Interframe Space (PIFS) is used for high priority access tothe medium during the contention-free period. The point coordinator inthe access point connected to backbone network, controls thepriority-based Point Coordination Function (PCF) to dictate whichstations in cell can gain access to the medium. The point coordinator inthe access point sends a contention-free poll frame to a station,granting the station permission to transmit a single frame to anydestination. All other stations in the cell can only transmit duringcontention-free period if the point coordinator grants them access tothe medium. The end of the contention-free period is signaled by thecontention-free end frame sent by the point coordinator, which occurswhen time expires or when the point coordinator has no further frames totransmit and no stations to poll.

The distributed coordination function (DCF) Interframe Space (DIFS) isused for transmitting low priority data frames during thecontention-based period. The DIFS spacing delays the transmission oflower priority frames to occur later than the priority-basedtransmission frames. An Extended Interframe Space (EIFS) goes beyond thetime of a DIFS interval, as a waiting period when a bad receptionoccurs. The EIFS interval provides enough time for the receiving stationto send an acknowledgment (ACK) frame.

During the contention-based period, the distributed coordinationfunction (DCF) uses the Carrier-Sense Multiple Access With CollisionAvoidance (CSMA/CA) contention-based protocol, which is similar to IEEE802.3 Ethernet. The CSMA/CA protocol minimizes the chance of collisionsbetween stations sharing the medium, by waiting a random backoffinterval, if the station's sensing mechanism indicates a busy medium.The period of time immediately following traffic on the medium is whenthe highest probability of collisions occurs, especially where there ishigh utilization. Once the medium is idle, CSMA/CA protocol causes eachstation to delay its transmission by a random backoff time, therebyminimizing the chance it will collide with those from other stations.

The CSMA/CA protocol computes the random backoff time as the product ofa constant, the slot time, times a pseudo-random number RN which has arange of values from zero to a collision window CW. The value of thecollision window for the first try to access the network is CW1, whichyields the first try random backoff time. If the first try to access thenetwork by a station fails, then the CSMA/CA protocol computes a new CWby doubling the current value of CW as CW2=CW1 times 2. The value of thecollision window for the second try to access the network is CW2, whichyields the second try random backoff time. This process by the CSMA/CAprotocol of increasing the delay before transmission is called binaryexponential backoff. The reason for increasing CW is to minimizecollisions and maximize throughput for both low and high networkutilization. Stations with low network utilization are not forced towait very long before transmitting their frame. On the first or secondattempt, a station will make a successful transmission. However, if theutilization of the network is high, the CSMA/CA protocol delays stationsfor longer periods to avoid the chance of multiple stations transmittingat the same time. If the second try to access the network fails, thenthe CSMA/CA protocol computes a new CW by again doubling the currentvalue of CW as CW3=CW1 times 4. The value of the collision window forthe third try to access the network is CW3, which yields the third tryrandom backoff time. The value of CW increases to relatively high valuesafter successive retransmissions, under high traffic loads. Thisprovides greater transmission spacing between stations waiting totransmit.

Collision Avoidance Techniques

Four general collision avoidance approaches have emerged: [1] CarrierSense Multiple Access (CSMA) [see, F. Tobagi and L. Kleinrock, “PacketSwitching in Radio Channels: Part I—Carrier Sense Multiple Access Modelsand their Throughput Delay Characteristics”, IEEE Transactions onCommunications, Vol 23, No 12, Pages 1400-1416, 1975], [2] MultipleAccess Collision Avoidance (MACA) [see, P. Karn, “MACA—A New ChannelAccess Protocol for Wireless Ad-Hoc Networks”, Proceedings of theARRL/CRRL Amateur Radio Ninth Computer Networking Conference, Pages134-140, 1990.], [3] their combination CSMA/CA, and [4] collisionavoidance tree expansion.

CSMA allows access attempts after sensing the channel for activity.Still, simultaneous transmit attempts lead to collisions, thus renderingthe protocol unstable at high traffic loads. The protocol also suffersfrom the hidden terminal problem.

The latter problem was resolved by the MACA protocol, which involves athree-way handshake [P. Karn, supra]. The origin node sends a request tosend (RTS) notice of the impending transmission. A response is returnedby the destination if the RTS notice is received successfully and theorigin node proceeds with the transmission. This protocol also reducesthe average delay as collisions are detected upon transmission of merelya short message, the RTS. With the length of the packet included in theRTS and echoed in the clear to send (CTS) messages, hidden terminals canavoid colliding with the transmitted message. However, this prevents theback-to-back re-transmission in case of unsuccessfully transmittedpackets. A five-way handshake MACA protocol provides notification tocompeting sources of the successful termination of the transmission.[see, V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, “MACAW: Amedia access protocol for wireless LANs, SIGCOMM '94, Pages 212-225,ACM, 1994.]

CSMA and MACA are combined in CSMA/CA, which is MACA with carriersensing, to give better performance at high loads. A four-way handshakeis employed in the basic contention-based access protocol used in theDistributed Coordination Function (DCF) of the IEEE 802.11 Standard forWireless LANs. [see, IEEE Standards Department, D3, “Wireless MediumAccess Control and Physical Layer WG,” IEEE Draft Standard P802.11Wireless LAN, Jan 1996.]

Collisions can be avoided by splitting the contending terminals beforetransmission is attempted. In the pseudo-Bayesian control method, eachterminal determines whether it has permission to transmit using a randomnumber generator and a permission probability “p” that depends on theestimated backlog. [see, R. L. Rivest, “Network control by BayesianBroadcast”, IEEE Trans. Inform. Theory, Vol IT 25, pp. 505-515, Sept1979]

To resolve collisions, subsequent transmission attempts are typicallystaggered randomly in time using the following two approaches: binarytree and binary exponential backoff.

Upon collision, the binary tree method requires the contending nodes toself-partition into two groups with specified probabilities. Thisprocess is repeated with each new collision. The order in whichcontending nodes transmit is determined either by serial or parallelresolution of the tree. [see, J. L. Massey, “Collision-resolutionalgorithms and random-access communications”, in Multi-UserCommunication Systems, G. Longo (ed.), CISM Courses and Lectures No.265. New York: Springer 1982, pp.73-137.]

In the binary exponential backoff approach, a backoff counter tracks thenumber of pauses and hence the number of completed transmissions beforea node with pending packets attempts to seize the channel. A contendingnode initializes its backoff counter by drawing a random value, giventhe backoff window size. Each time the channel is found idle, thebackoff counter is decreased and transmission is attempted uponexpiration of the backoff counter. The window size is doubled every timea collision occurs, and the backoff countdown starts again. [see, A.Tanenbaum, Computer Networks, 3^(rd) ed., Upper Saddle River, N.J.,Prentice Hall, 1996] The Distributed Coordination Function (DCF) of theIEEE 802.11 Standard for Wireless LANs employs a variant of thiscontention resolution scheme, a truncated binary exponential backoff,starting at a specified window and allowing up to a maximum backoffrange below which transmission is attempted. [IEEE Standards Department,D3, supra] Different backoff counters may be maintained by a contendingnode for traffic to specific destinations. [Bharghavan, supra]

In the IEEE 802.11 Standard, the channel is shared by a centralizedaccess protocol, the Point Coordination Function (PCF), which providescontention-free transfer based on a polling scheme controlled by theaccess point (AP) of a basic service set (BSS). [IEEE StandardsDepartment, D3, supra] The centralized access protocol gains control ofthe channel and maintains control for the entire contention-free periodby waiting a shorter time between transmissions than the stations usingthe Distributed Coordination Function (DCF) access procedure. Followingthe end of the contention-free period, the DCF access procedure begins,with each station contending for access using the CSMA/CA method.

The 802.11 MAC Layer provides both contention and contention-free accessto the shared wireless medium. The MAC Layer uses various MAC frametypes to implement its functions of MAC management, control, and datatransmission. Each station and access point on an 802.11 wireless LANimplements the MAC Layer service, which enables stations to exchangepackets. The results of sensing the channel to determine whether themedium is busy or idle, are sent to the MAC coordination function of thestation. The MAC coordination also carries out a virtual carrier senseprotocol based on reservation information found in the Duration Field ofall frames. This information announces to all other stations, thesending station's impending use of the medium. The MAC coordinationmonitors the Duration Field in all MAC frames and places thisinformation in the station's Network Allocation Vector (NAV) if thevalue is greater than the current NAV value. The NAV operates similarlyto a timer, starting with a value equal to the Duration Field of thelast frame transmission sensed on the medium, and counting down to zero.After the NAV reaches zero, the station can transmit, if its physicalsensing of the channel indicates a clear channel.

At the beginning of a contention-free period, the access point sensesthe medium, and if it is idle, it sends a Beacon packet to all stations.The Beacon packet contains the length of the contention-free interval.The MAC coordination in each member station places the length of thecontention-free interval in the station's Network Allocation Vector(NAV), which prevents the station from taking control of the mediumuntil the end of the contention-free period. During the contention-freeperiod, the access point can send a polling message to a member station,enabling it to send a data packet to any other station in the BSSwireless cell.

Quality Of Service (QoS)

Quality of service (QoS) is a measure of service quality provided to acustomer. The primary measures of QoS are message loss, message delay,and network availability. Voice and video applications have the mostrigorous delay and loss requirements. Interactive data applications suchas Web browsing have less restrained delay and loss requirements, butthey are sensitive to errors. Non-real-time applications such as filetransfer, Email, and data backup operate acceptably across a wide rangeof loss rates and delay. Some applications require a minimum amount ofcapacity to operate at all, for example, voice and video. Many networkproviders guarantee specific QoS and capacity levels through the use ofService-Level Agreements (SLAs). An SLA is a contract between anenterprise user and a network provider that specifies the capacity to beprovided between points in the network that must be delivered with aspecified QoS. If the network provider fails to meet the terms of theSLA, then the user may be entitled a refund. The SLA is typicallyoffered by network providers for private line, frame relay, ATM, orInternet networks employed by enterprises.

The transmission of time-sensitive and data application traffic over apacket network imposes requirements on the delay or delay jitter, andthe error rates realized; these parameters are referred to genericallyas the QoS (Quality of Service) parameters. Prioritized packetscheduling, preferential packet dropping, and bandwidth allocation areamong the techniques available at the various nodes of the network,including access points, that enable packets from different applicationsto be treated differently, helping achieve the different quality ofservice objectives. Such techniques exist in centralized and distributedvariations. The concern herein is with distributed mechanisms formultiple access in cellular packet networks or wireless ad hoc networks.

Management of contention for the shared transmission medium must reflectthe goals sought for the performance of the overall system. Forinstance, one such goal would be the maximization of goodput (the amountof good data transmitted as a fraction of the channel capacity) for theentire system, or of the utilization efficiency of the RF spectrum;another is the minimization of the worst-case delay. As multiple typesof traffic with different performance requirements are combined intopacket streams that compete for the same transmission medium, amulti-objective optimization is required.

Ideally, one would want a multiple access protocol that is capable ofeffecting packet transmission scheduling as close to the optimalscheduling as possible, but with distributed control. Distributedcontrol implies both some knowledge of the attributes of the competingpacket sources and limited control mechanisms.

To apply any scheduling algorithm in random multiple access, a mechanismmust exist that imposes an order in which packets will seize the medium.For distributed control, this ordering must be achieved independently,without any prompting or coordination from a control node. Only if thereis a reasonable likelihood that packet transmissions will be orderedaccording to the scheduling algorithm, can one expect that thealgorithm's proclaimed objective will be attained.

The above cited, copending patent application by Mathilde Benveniste,entitled “Tiered Contention Multiple Access (TCMA): A Method forPriority-Based Shared Channel Access”, describes the Tiered ContentionMultiple Access (TCMA) distributed medium access protocol that schedulestransmission of different types of traffic based on their QoS servicequality specifications. This protocol makes changes to the contentionwindow following the transmission of a frame, and therefore is alsocalled Extended-DCF (E-DCF). During the contention window, the variousstations on the network contend for access to the network. To avoidcollisions, the MAC protocol requires that each station first wait for arandomly-chosen time period, called an arbitration time. Since thisperiod is chosen at random by each station, there is less likelihood ofcollisions between stations. TCMA uses the contention window to givehigher priority to some stations than to others. Assigning a shortcontention window to those stations that should have higher priorityensures that in most cases, the higher-priority stations will be able totransmit ahead of the lower-priority stations. TCMA schedulestransmission of different types of traffic based on their QoS servicequality specifications. As seen in FIG. 1, which depicts the tieredcontention mechanism, a station cannot engage in backoff countdown untilthe completion of an idle period of length equal to its arbitrationtime.

The above cited, copending patent application by Mathilde Benvenistealso applies TCMA to the use of the wireless access point as a trafficdirector. This application of the TCMA protocol is called the hybridcoordination function (HCF). In HCF, the access point uses a pollingtechnique as the traffic control mechanism. The access point sendspolling packets to a succession of stations on the network. Theindividual stations can reply to the poll with a packet that containsnot only the response, but also any data that needs to be transmitted.Each station must wait to be polled. The access point establishes apolling priority based on the QoS priority of each station.

What is needed in the prior art is a way to apply the hybridcoordination function (HCF) to wireless cells that have overlappingaccess points contending for the same medium.

SUMMARY OF THE INVENTION

In accordance with the invention, the Tiered Contention Multiple Access(TCMA) protocol is applied to wireless cells which have overlappingaccess points contending for the same medium. Quality of Service (QoS)support is provided to overlapping access points to scheduletransmission of different types of traffic based on the service qualityspecifications of the access points.

The inventive method reduces interference in a medium betweenoverlapping wireless LAN cells, each cell including an access pointstation and a plurality of member stations. In accordance with theinvention, the method assigns to a first access point station in a firstwireless LAN cell, a first scheduling tag. The scheduling tag has avalue that determines an accessing order for the cell in a transmissionframe, with respect to the accessing order of other wireless cells. Thescheduling tag value is deterministically set. The scheduling tag valuecan be permanently assigned to the access point by its manufacturer, itcan be assigned by the network administrator at network startup, it canbe assigned by a global processor that coordinates a plurality ofwireless cells over a backbone network, it can be drawn from a pool ofpossible tag values during an initial handshake negotiation with otherwireless stations, or it can be cyclically permuted in real-time, on aframe-by-frame basis, from a pool of possible values, coordinating thatcyclic permutation with that of other access points in other wirelesscells.

An access point station in a wireless cell signals the beginning of anintra-cell contention-free period for member stations in its cell bytransmitting a beacon packet. The duration of the intra-cellcontention-free period is deterministically set. The member stations inthe cell store the intra-cell contention-free period value as a NetworkAllocation Vector (NAV). Each member station in the cell decrements thevalue of the NAV in a manner similar to other backoff time values,during which it will delay accessing the medium.

In accordance with the invention, the method assigns to the first accesspoint station, a first inter-cell contention-free period value, whichgives notice to any other cell receiving the beacon packet, that thefirst cell has seized the medium for the period of time represented bythe value. The inter-cell contention-free period value isdeterministically set. Further in accordance with the invention, anystation receiving the beacon packet immediately broadcasts acontention-free time response (CFTR) packet containing a copy of thefirst inter-cell contention-free period value. In this manner, thenotice is distributed to a second access point station in anoverlapping, second cell. The second access point stores the firstinter-cell contention-free period value as an Inter-BSS NetworkAllocation Vector (IBNAV). The second access point decrements the valueof IBNAV in a manner similar to other backoff time values, during whichit will delay accessing the medium.

Still further in accordance with the invention, the method also assignsto first member stations in the first cell, a first shorter backoffvalue for high Quality of Service (QoS) data and a first longer backoffvalue for lower QoS data. The backoff time is the interval that a memberstation waits after the expiration of the contention-free period, beforethe member station contends for access to the medium. Since more thanone member station in a cell may be competing for access, the actualbackoff time for a particular station can be selected as one of severalpossible values. In one embodiment, the actual backoff time for eachparticular station is deterministically set, so as to reduce the lengthof idle periods. In another embodiment, the actual backoff time for eachparticular station is randomly drawn from a range of possible valuesbetween a minimum delay interval to a maximum delay interval. The rangeof possible backoff time values is a contention window. The backoffvalues assigned to a cell may be in the form of a specified contentionwindow. High QoS data is typically isochronous data such as streamingvideo or audio data that must arrive at its destination at regularintervals. Low QoS data is typically file transfer data and email, whichcan be delayed in its delivery and yet still be acceptable. The TieredContention Multiple Access (TCMA) protocol coordinates the transmissionof packets within a cell, so as to give preference to high QoS data overlow QoS data, to insure that the required quality of service ismaintained for each type of data.

The method similarly assigns to a second access point station in asecond wireless LAN cell that overlaps the first sell, a secondcontention-free period value longer than the first contention-freeperiod value. The method also assigns to second member stations in thesecond cell, a second shorter backoff value for high QoS data and asecond longer backoff value for lower QoS data. The first and secondcells are considered to be overlapped when one or more stations in thefirst cell inadvertently receive packets from member stations or theaccess point of the other cell. The invention reduces the interferencebetween the overlapped cells by coordinating the timing of theirrespective transmissions, while maintaining the TCMA protocol'spreference for the transmission of high QoS data over low QoS data ineach respective cell.

During the operation of two overlapped cells, the method transmits afirst beacon packet including the intra-cell contention-free periodvalue (the increment to the NAV) and inter-cell contention-free periodvalue (the CFTR), from the first access point to the first memberstations in the first cell. The beacon packet is received by the memberstations of the first cell and can be inadvertently received by at leastone overlapped member station of the second cell. Each member station inthe first cell increments its NAV with the intra-cell contention-freeperiod value and stores the inter-cell contention-free period value asthe CFTR.

In accordance with the invention, each station that receives the firstbeacon packet, immediately responds by transmitting a firstcontention-free time response (CFTR) packet that contains a copy of theinter-cell contention-free period value (CFTR). A CFTR packet istransmitted from the first member stations in the first cell and also bythe overlapped member stations of the second cell. The effect of thetransmission of CFTR packets from member stations in the second cell isto alert the second access point and the second member stations in thesecond cell, that the medium has been seized by the first access pointin the first cell. When the second access point in the second cellreceives the CFTR packet it stores the a copy of the inter-cellcontention-free period value as the IBNAV.

Similar to a station's Network Allocation Vector (NAV), a first IBNAV isset at the second access point to indicate the time the medium will befree again. Also similar to the NAV, the first IBNAV is decremented witheach succeeding slot, similar to the decrementing of other backofftimes. When the second access point receives the first IBNAVrepresenting the first cell's contention-free period value, the secondaccess point must respect the first IBNAV value and delay transmittingits beacon packet and the exchange of other packets in the second celluntil the expiration of the received, first IBNAV.

When the second access point has decremented the first IBNAV to zero,the second access point transmits its second beacon packet including itssecond contention-free period values of NAV and a second IBNAV, to thesecond member stations in the second cell. Each station that receivesthe second beacon packet immediately responds by transmitting a secondcontention-free time response (CFTR) packet that contains a copy of thesecond IBNAV inter-cell contention-free period value. The second CFTRpacket is transmitted from the second member stations in the second celland also by the overlapped member stations of the first cell. The effectof the transmission of the second CFTR packets from overlapped memberstations in the first cell is to alert the first access point and thefirst member stations in the first cell, that the medium has been seizedby the second access point in the second cell. When the first accesspoint in the first cell receives the CFTR packet it stores the a copy ofthe second IBNAV inter-cell contention-free period value, to indicatethe time the medium will be free again. The second IBNAV is decrementedwith each succeeding frame, similar to the decrementing of other backofftimes.

The second member stations in the second cell wait for completion of thecount down of their NAVs to begin the TCMA protocol of counting down thesecond shorter backoff for high QoS data and then transmitting secondhigh QoS data packets.

Meanwhile, the first access point in the first cell waits for completionof the count down of the second IBNAV inter-cell contention-free periodbefore starting the countdown of its own NAV for its own intra-cellcontention-free period. The first member stations in the first cell waitfor the count down of their NAVs, to begin the TCMA protocol of countingdown the first longer backoff for low QoS data and then transmittingfirst low QoS data.

Meanwhile the second member stations are waiting for the TCMA protocolof counting down the second longer backoff for lower QoS data beforetransmitting the second lower QoS data.

In this manner, interference in a medium between overlapping wirelessLAN cells is reduced.

Potential collisions between cells engaged in centralized access can beaverted or resolved by the TCMA protocol. In accordance with theinvention, deterministically set backoff delays are used, which tend toreduce the length of the idle periods. The possibility of coincident oroverlapping contention-free periods between neighboring cells iseliminated through the use of an “interference sensing” method employinga new frame.

The invention enables communication of channel occupancy information toneighboring access points. When a beacon packet is transmitted, andbefore transmission of any other data or polling packets, all stationshearing the beacon will respond by sending a frame, the contention-freetime response (CFTR), that will contain the duration of thecontention-free period found in the beacon. An access point inneighboring cells, or stations attempting contention-based channelaccess, which receive this message from a station in the celloverlapping region, are thus alerted that the channel has been seized byan access point. Similar to a station's Network Allocation Vector (NAV),an Inter-Cell Network Allocation Vector at the access point accordinglyindicates when the time the channel will be free again. Unless theInter-Cell Network Allocation Vector is reset, the access point willdecrease its backoff value only after the expiration of the Inter-CellNetwork Allocation Vector, according to the backoff countdown rules.

In another aspect of the invention, potential collisions betweendifferent access points engaged in centralized access can be averted orresolved by using deterministic backoff delays, which avoid collisionsbetween access points, and eliminate gaps between consecutivepoll/response exchanges or contention-free bursts (CFBs) between theaccess point and its associated stations.

The resulting invention applies the Tiered Contention Multiple Access(TCMA) protocol to wireless cells which have overlapping access pointscontending for the same medium.

DESCRIPTION OF THE FIGURES

FIG. 1 depicts the tiered contention mechanism.

FIGS. 1A through 1J show the interaction of two wireless LAN cells whichhave overlapping access points contending for the same medium, inaccordance with the invention.

FIG. 1K shows a timing diagram for the interaction of two wireless LANcells in FIGS. 1A through 1J, in accordance with the invention.

FIG. 1L shows the IEEE 802.11 packet structure for a beacon packet,including the increment to the NAV period and the CFTR period, inaccordance with the invention.

FIG. 1M shows the IEEE 802.11 packet structure for a CFTR packet,including the CFTR period, in accordance with the invention.

FIG. 2 illustrates the ordering of transmissions from three groups ofBSSs.

FIG. 3 illustrates how three interfering BSSs share the same channel fortwo consecutive frames.

FIG. 4 illustrates how three interfering BSSs, each with two types oftraffic of different priorities, share the same channel in twoconsecutive frames.

FIG. 5 illustrates the possible re-use of tags.

FIG. 6 illustrates the deterministic post-backoff.

FIG. 7 shows the relationships of repeating sequences of CFBs.

FIG. 8 illustrates the role of pegging in a sequence of CFBs by threeoverlapping access points.

FIG. 9 illustrates the start-up procedure for a new access point, HC2,given an existing access point, HC1.

FIG. 10 shows the relationship of repeating sequences of Tier I CFBs.

FIG. 11 illustrates the start-up procedure for a new access point, HC2,given an existing access point, HC1.

DISCUSSION OF THE PREFERRED EMBODIMENT

The invention disclosed broadly relates to telecommunications methodsand more particularly relates to Quality-of-Service (QoS) management inmultiple access packet networks. Several protocols, either centralizedor distributed can co-exist on the same channel through the TieredContention Multiple Access method. The proper arbitration time to beassigned to the centralized access protocol must satisfy the followingrequirements: (i) the centralized access protocol enjoys top priorityaccess, (ii) once the centralized protocol seizes the channel, itmaintains control until the contention-free period ends, (iii) theprotocols are backward compatible, and (iv) Overlapping Basic ServiceSets (OBSSs) engaged in centralized-protocol can share the channelefficiently.

In accordance with the invention, the Tiered Contention Multiple Access(TCMA) protocol is applied to wireless cells which have overlappingaccess points contending for the same medium. Quality of Service (QoS)support is provided to overlapping access points to scheduletransmission of different types of traffic based on the service qualityspecifications of the access points.

The inventive method reduces interference in a medium betweenoverlapping wireless LAN cells, each cell including an access pointstation and a plurality of member stations. FIGS. 1A through 1J show theinteraction of two wireless LAN cells which have overlapping accesspoints contending for the same medium, in accordance with the invention.The method assigns to a first access point station in a first wirelessLAN cell, a first scheduling tag. The scheduling tag has a value thatdetermines an accessing order for the cell in a transmission frame, withrespect to the accessing order of other wireless cells. The schedulingtag value is deterministically set. The scheduling tag value can bepermanently assigned to the access point by its manufacturer, it can beassigned by the network administrator at network startup, it can beassigned by a global processor that coordinates a plurality of wirelesscells over a backbone network, it can be drawn from a pool of possibletag values during an initial handshake negotiation with other wirelessstations, or it can be cyclically permuted in real-time, on aframe-by-frame basis, from a pool of possible values, coordinating thatcyclic permutation with that of other access points in other wirelesscells.

The interaction of the two wireless LAN cells 100 and 150 in FIGS. 1Athrough 1J is shown in the timing diagram of FIG. 1K. The timing diagramof FIG. 1K begins at instant T0, goes to instant T9, and includesperiods P1 through P8, as shown in the figure. The various packetsdiscussed below are also shown in FIG. 1K, placed at their respectivetimes of occurrence. An access point station in a wireless cell signalsthe beginning of an intra-cell contention-free period for memberstations in its cell by transmitting a beacon packet. FIG. 1A showsaccess point 152 of cell 150 connected to backbone network 160,transmitting the beacon packet 124. In accordance with the invention,the beacon packet 124 includes two contention-free period values, thefirst is the Network Allocation Vector (NAV) (or alternately itsincremental value ΔNAV), which specifies a period value P3 for theintra-cell contention-free period for member stations in its own cell.Member stations within the cell 150 must wait for the period P3 beforebeginning the Tiered Contention Multiple Access (TCMA) procedure, asshown in FIG. 1K. The other contention-free period value included in thebeacon packet 124 is the Inter-BSS Network Allocation Vector (IBNAV),which specifies the contention-free time response (CFTR) period P4. Thecontention-free time response (CFTR) period P4 gives notice to any othercell receiving the beacon packet, such as cell 100, that the first cell150 has seized the medium for the period of time represented by thevalue P4.

The beacon packet 124 is received by the member stations 154A (with alow QoS requirement 164A) and 154B (with a high QoS requirement 164B) inthe cell 150 during the period from T1 to T2. The member stations 154Aand 154B store the value of ΔNAV =P3 and begin counting down that valueduring the contention free period of the cell 150. The duration of theintra-cell contention-free period ΔNAV=P3 is deterministically set. Themember stations in the cell store the intra-cell contention-free periodvalue P3 as the Network Allocation Vector (NAV). Each member station inthe cell 150 decrements the value of the NAV in a manner similar toother backoff time values, during which it will delay accessing themedium. FIG. 1L shows the IEEE 802.11 packet structure 260 for thebeacon packet 124 or 120, including the increment to the NAV period andthe CFTR period. The beacon packet structure 260 includes fields 261 to267. Field 267 specifies the ΔNAV value of P3 and the CFTR value of P4.In accordance with the invention, the method assigns to the first accesspoint station, a first inter-cell contention-free period value, whichgives notice to any other cell receiving the beacon packet, that thefirst cell has seized the medium for the period of time represented bythe value. The inter-cell contention-free period value isdeterministically set.

Further in accordance with the invention, any station receiving thebeacon packet 124 immediately rebroadcasts a contention-free timeresponse (CFTR) packet 126 containing a copy of the first inter-cellcontention-free period value P4. The value P4 specifies the Inter-BSSNetwork Allocation Vector (IBNAV), i.e., the contention-free timeresponse (CFTR) period that the second access point 102 must wait, whilethe first cell 150 has seized the medium. FIG. 1B shows overlap station106 in the region of overlap 170 transmitting the CFTR packet 126 tostations in both cells 100 and 150 during the period from T1 to T2. FIG.1M shows the IEEE 802.11 packet structure 360 for a CFTR packet 126 or122, including the CFTR period. The CFTR packet structure 360 includesfields 361 to 367. Field 367 specifies the CFTR value of P4. In thismanner, the notice is distributed to the second access point station 102in the overlapping, second cell 100.

FIG. 1C shows the point coordinator in access point 152 of cell 150controlling the contention-free period within the cell 150 by using thepolling packet 128 during the period from T2 to T3. In the mean time,the second access point 102 in the second cell 100 connected to backbonenetwork 110, stores the first inter-cell contention-free period value P4received in the CFTR packet 126, which it stores as the Inter-BSSNetwork Allocation Vector (IBNAV). The second access point 102decrements the value of IBNAV in a manner similar to other backoff timevalues, during which it will delay accessing the medium.

Still further in accordance with the invention, the method uses theTiered Contention Multiple Access (TCMA) protocol to assign to firstmember stations in the first cell 150, a first shorter backoff value forhigh Quality of Service (QoS) data and a first longer backoff value forlower QoS data. FIG. 1D shows the station 154B in the cell 150, having ahigh QoS requirement 164B, decreasing its High QoS backoff period tozero and beginning TCMA contention to transmit its high QoS data packet130 during the period from T3 to T4. The backoff time is the intervalthat a member station waits after the expiration of the contention-freeperiod P3, before the member station 154B contends for access to themedium. Since more than one member station in a cell may be competingfor access, the actual backoff time for a particular station can beselected as one of several possible values. In one embodiment, theactual backoff time for each particular station is deterministicallyset, so as to reduce the length of idle periods. In another embodiment,the actual backoff time for each particular station is randomly drawnfrom a range of possible values between a minimum delay interval to amaximum delay interval. The range of possible backoff time values is acontention window. The backoff values assigned to a cell may be in theform of a specified contention window. High QoS data is typicallyisochronous data such as streaming video or audio data that must arriveat its destination at regular intervals. Low QoS data is typically filetransfer data and email, which can be delayed in its delivery and yetstill be acceptable. The Tiered Contention Multiple Access (TCMA)protocol coordinates the transmission of packets within a cell, so as togive preference to high QoS data over low QoS data, to insure that therequired quality of service is maintained for each type of data.

The method similarly assigns to the second access point 102 station inthe second wireless LAN cell 100 that overlaps the first sell 150, asecond contention-free period value CFTR=P7 longer than the firstcontention-free period value CFTR=P4. FIG. 1E shows the second accesspoint 102 in the cell 100 transmitting its beacon packet 120 includingits contention-free period values of NAV (P6) and IBNAV (P7), to themember stations 104A (with a low QoS requirement 114A), 104B (with ahigh QoS requirement 114B) and 106 in the cell 100 during the periodfrom T4 to T5. FIG. 1F shows that each station, including the overlapstation 106, that receives the second beacon packet 120, immediatelyresponds by retransmitting a second contention-free time response (CFTR)packet 122 that contains a copy of the second inter-cell contention-freeperiod value P7 during the period from T4 to T5.

FIG. 1G shows the point coordinator in access point 102 of cell 100controlling the contention-free period within cell 100 using the pollingpacket 132 during the period from T5 to T6.

The method uses the Tiered Contention Multiple Access (TCMA) protocol toassign to second member stations in the second cell 100, a secondshorter backoff value for high QoS data and a second longer backoffvalue for lower QoS data. FIG. 1H shows the station 104B in the cell100, having a high QoS requirement 114B, decreasing its High QoS backoffperiod to zero and beginning TCMA contention to transmit its high QoSdata packet 134 during the period from T6 to T7. FIG. 1I shows the firstmember stations 154A and 154B in the first cell 150 waiting for thecount down of their NAVs, to begin the TCMA protocol of counting downthe first longer backoff for low QoS data and then transmitting firstlow QoS data 136 during the period from T7 to T8. FIG. 1J shows thesecond member stations 104A, 104B, and 106 are waiting for the TCMAprotocol of counting down the second longer backoff for lower QoS databefore transmitting the second lower QoS data 138 during the period fromT8 to T9.

The first and second cells are considered to be overlapped when one ormore stations in the first cell can inadvertently receive packets frommember stations or the access point of the other cell. The inventionreduces the interference between the overlapped cells by coordinatingthe timing of their respective transmissions, while maintaining the TCMAprotocol's preference for the transmission of high QoS data over low QoSdata in each respective cell.

During the operation of two overlapped cells, the method in FIG. 1Atransmits a first beacon packet 124 including the intra-cellcontention-free period value (the increment to the NAV) and inter-cellcontention-free period value (the CFTR), from the first access point 152to the first member stations 154B and 154A in the first cell 150. Thebeacon packet is received by the member stations of the first cell andinadvertently by at least one overlapped member station 106 of thesecond cell 100. Each member station 154B and 154A in the first cellincrements its NAV with the intra-cell contention-free period value P3and stores the inter-cell contention-free period value P4 as the CFTR.

In accordance with the invention, each station that receives the firstbeacon packet 124, immediately responds by transmitting a firstcontention-free time response (CFTR) packet 126 in FIG. 1B that containsa copy of the inter-cell contention-free period P4 value (CFTR). A CFTRpacket 126 is transmitted from the first member stations 154B and 154Ain the first cell 150 and also by the overlapped member stations 106 ofthe second cell 100. The effect of the transmission of CFTR packets 126from member stations 106 in the second cell 100 is to alert the secondaccess point 102 and the second member stations 104A and 104B in thesecond cell 100, that the medium has been seized by the first accesspoint 152 in the first cell 150. When the second access point 102 in thesecond cell 100 receives the CFTR packet 126 it stores the a copy of theinter-cell contention-free period value P4 as the IBNAV.

Similar to a station's Network Allocation Vector (NAV), an IBNAV is setat the access point to indicate the time the medium will be free again.Also similar to the NAV, the IBNAV is decremented with each succeedingslot, similar to the decrementing of other backoff times. When thesecond access point receives a new IBNAV representing the first cell'scontention-free period value, then the second access point must respectthe IBNAV value and delay transmitting its beacon packet and theexchange of other packets in the second cell until the expiration of thereceived, IBNAV.

Later, as shown in FIG. 1E, when the second access point 102 transmitsits second beacon packet 120 including its second contention-free periodvalues of NAV (P6) and IBNAV (P7), to the second member stations 104A,104B and 106 in the second cell 100, each station that receives thesecond beacon packet, immediately responds by transmitting a secondcontention-free time response (CFTR) packet 122 in FIG. 1F, thatcontains a copy of the second inter-cell contention-free period valueP7. A CFTR packet 122 is transmitted from the second member stations104A, 104B and overlapped station 106 in the second cell and also by theoverlapped member stations of the first cell. The effect of thetransmission of CFTR packets from overlapped member station 106 is toalert the first access point 152 and the first member stations 145A and154B in the first cell 150, that the medium has been seized by thesecond access point 102 in the second cell 100. When the first accesspoint 152 in the first cell 150 receives the CFTR packet 122 it storesthe a copy of the second inter-cell contention-free period value P7 asan IBNAV, to indicate the time the medium will be free again. The IBNAVis decremented with each succeeding slot, similar to the decrementing ofother backoff times.

The second member stations 104A, 104B, and 106 in the second cell 100wait for completion of the count down of their NAVs to begin the TCMAprotocol of counting down the second shorter backoff for high QoS dataand then transmitting second high QoS data packets, as shown in FIGS. 1Gand 1H.

Meanwhile, the first access point 152 in the first cell 150 waits forcompletion of the count down of the second inter-cell contention-freeperiod P7 in its IBNAV in FIGS. 1G and 1H before starting the countdownof its own NAV for its own intra-cell contention-free period. The firstmember stations 154A and 154B in the first cell 150 wait for the countdown of their NAVs, to begin the TCMA protocol of counting down thefirst longer backoff for low QoS data and then transmitting first lowQoS data in FIG. 1I.

Meanwhile the second member stations 104A, 104B, and 106 are waiting forthe TCMA protocol of counting down the second longer backoff for lowerQoS data before transmitting the second lower QoS data 138 in FIG. 1J.

In this manner, interference in a medium between overlapping wirelessLAN cells is reduced.

DETAILED DESCRIPTION OF THE INVENTION

TCMA can accommodate co-existing Extended Distributed CoordinationFunction (E-DCF) and centralized access protocols. In order to ensurethat the centralized access protocol operating under Hybrid CoordinationFunction (HCF) is assigned top priority access, it must have theshortest arbitration time. Its arbitration time is determined byconsidering two additional requirements: uninterrupted control of thechannel for the duration of the contention-free period, and backwardcompatibility.

Uninterrupted Contention-Free Channel Control

The channel must remain under the control of the centralized accessprotocol until the contention-free period is complete once it has beenseized by the centralized access protocol. For this, it is sufficientthat the maximum spacing between consecutive transmissions exchanged inthe centralized access protocol, referred to as the central coordinationtime (CCT), be shorter than the time the channel must be idle before astation attempts a contention-based transmission following the end of abusy-channel time interval. The centralized access protocol has a CCTequal to the Priority Interframe Space (PIFS). Hence, no station mayaccess the channel by contention, using either the distributedcoordination function (DCF) or Extended-DCF (E-DCF) access procedure,before an idle period of length of the DCF Interframe Space (DIFS)equaling PIFS+1 (slot time) following the end of a busy-channel timeinterval. This requirement is met by DCF. For E-DCF, it would besufficient for the Urgency Arbitration Time (UAT) of a class j, UATj, tobe greater than PIFS for all classes j>1.

Backward Compatibility

Backward compatibility relates to the priority treatment of traffichandled by enhanced stations (ESTAs) as compared to legacy stations(STAs). In addition to traffic class differentiation, the ESTAs mustprovide certain traffic classes with higher or equal priority accessthan that provided by the STAs. That means that certain traffic classesshould be assigned a shorter arbitration times than DIFS, the de factoarbitration time of legacy stations.

Because the time in which the “clear channel assessment” (CCA) functioncan be completed is set at the minimum attainable for the IEEE 802.11physical layer (PHY) specification, the arbitration times of any twoclasses of different priority would have to be separated by at least one“time slot”. This requirement implies that the highest priority trafficclass would be required to have an arbitration time equal to DIFS−1(slottime)=PIFS.

Though an arbitration time of PIFS appears to fail meeting therequirement for uninterrupted control of the channel during thecontention-free period, it is possible for an ESTA to access the channelby E-DCF using an arbitration time of PIFS and, at the same time, allowpriority access to the centralized access protocol at PIFS. This isachieved as follows. Contention-based transmission is restricted tooccur after a DIFS idle period following the end of a busy channelperiod by ensuring that the backoff value of such stations is drawn froma random distribution with lower bound that is at least 1. Given thatall backlogged stations resume backoff countdown after a busy-channelinterval with a residual backoff of at least 1, an ESTA will attempttransmission following completion of the busy interval only after anidle period equal to PIFS+1 (slot time)=DIFS. This enables thecentralized access protocol to maintain control of the channel withoutcolliding with contention-based transmissions by ESTAs attempting toaccess the channel using E-DCF.

To see that the residual backoff value of a backlogged station will begreater than or equal to 1 whenever countdown is resumed at the end of abusy channel period, consider a station with a backoff value m>0. Thestation will decrease its residual backoff value by 1 following eachtime slot during which the channel remains idle. If m reaches 0 beforecountdown is interrupted by a transmission, the station will attempttransmission. The transmission will either fail, leading to a newbackoff being drawn, or succeed. Therefore, countdown will be resumedafter the busy-channel period ends, only with a residual backoff of 1 orgreater. Consequently, if the smallest random backoff that can be drawnis 1 or greater, an ESTA will always wait for at least a DIFS idleinterval following a busy period before it attempts transmission.

Only one class can be derived with priority above legacy throughdifferentiation by arbitration time alone, by using the arbitration timeof PIFS. Multiple classes with that priority can be obtained bydifferentiation through other parameters, such as the parameters of thebackoff time distribution; e.g. the contention window size. For all theclasses so derived, a DIFS idle period will follow a busy channelinterval before the ESTA seizes the channel if the restriction isimposed that the backoff value of such stations be drawn from a randomdistribution with lower bound of at least 1.

Because PIFS is shorter than DIFS, the traffic classes with arbitrationtime equal to PIFS will have higher access priority than the trafficclasses with arbitration time equal to DIFS. As seen in FIG. 1, whichdepicts the tiered contention mechanism, a station cannot engage inbackoff countdown until the completion of an idle period of length equalto its arbitration time. Therefore, a legacy station will be unable toresume backoff countdown at the end of a busy-channel interval, if anESTA with arbitration time of PIFS has a residual backoff of 1.Moreover, a legacy station will be unable to transmit until allhigher-priority ESTAs with residual backoff of 1 have transmitted. Onlylegacy stations that draw a backoff value of 0 will transmit after aDIFS idle period, thus competing for the channel with the higherpriority stations. This occurs only with a probability less than 3 percent, since the probability of drawing a random backoff of 0 from therange [0,31] is equal to 1/32.

Top Priority for the Centralized Access Protocol

For the centralized access protocol to enjoy the highest priorityaccess, it must have an arbitration time shorter than PIFS by at least atime slot; that is, its arbitration time must equal PIFS−1 (slottime)=the Short Interframe Space (SIFS). As in the case of the highesttraffic priority classes for ESTAs accessing the channel by E-DCF, therandom backoff values for the beacon of the centralized access protocolmust be drawn from a range with a lower bound of at least 1. Using thesame reasoning as above, the centralized access protocol will nottransmit before an idle period less than PIFS=SIFS+1 (slot time), thusrespecting the inter-frame spacing requirement for a SIFS idle periodwithin frame exchange sequences. Consequently, the shorter arbitrationtime assigned to the centralized access protocol ensures that itaccesses the channel with higher priority than any station attemptingcontention-based access through E-DCF, while at the same time respectingthe SIFS spacing requirement.

It should be noted that while collisions are prevented between frameexchanges during the contention-free period, collisions are possibleboth between the beacons of centralized access protocols of differentBSSs located within interfering range [having coverage overlap], andbetween the beacon of a centralized access protocol and stationsaccessing the channel by contention using E-DCF. The probability of suchcollisions is low because higher priority nodes with residual backoffvalue m equal to 1 always seize the channel before lower priority nodes.Inter-access point collisions are resolved through the backoff procedureof TCMA.

Inter-Access Point Contention

Potential collisions between BSSs engaged in centralized access can beaverted or resolved by a backoff procedure. The complication arisinghere is that a random backoff delay could result in idle periods longerperiods than the SIFS+1 (slot time)=PIFS, which is what ensures priorityaccess to the centralized protocol over E-DCF traffic contention-basedtraffic. Hence, the collisions with contention-based traffic wouldoccur. Using short backoff windows in order to avoid this problem wouldincrease the collisions experienced. In accordance with the invention,deterministically set backoff delays are used, which tend to reduce thelength of the idle periods.

Another aspect of inter-BSS interference that affects the performance ofcentralized protocols adversely is the possible interruption with acollision of what starts as an interference-free poll/response exchangebetween the access point and its associated stations. The possibility ofcoincident or overlapping contention-free periods between neighboringBSSs is eliminated through the use of an “interference sensing” methodemploying a new frame.

Deterministic Backoff Procedure for the Centralized Access Protocol

A modified backoff procedure is pursued for the beacons of thecentralized access protocols. A backoff counter is employed in the sameway as in TCMA. But while the backoff delay in TCMA is selected randomlyfrom a contention window, in the case of the centralized access protocolbeacons, the backoff value is set deterministically.

Scheduling of packet transmission occurs once per frame, at thebeginning of the frame. Only the packets queued at the start of a framewill be transmitted in that frame. It is assumed that BSSs aresynchronized. A means for achieving such synchronization is through theexchange of messages relayed by boundary stations [stations in theoverlapping regions of neighboring BSSs].

The backoff delay is selected through a mechanism called “tagscheduling”. Tags, which are ordinal labels, are assigned to differentBSSs. BSSs that do not interfere with one another may be assigned thesame tag, while BSSs with the potential to interfere with one anothermust receive different tags. For each frame, the tags are ordered in away that is known a priori. This order represents the sequence in whichthe BSS with a given tag will access the channel in that frame. Thebackoff delay increases with the rank of the “tag” that has beenassigned to the BSS for the current frame, as tags are permuted to giveeach group of BSS with the same tag a fair chance at the channel. Forinstance, a cyclic permutation for three tags, t=1, 2, 3, would give thefollowing ordering: 1, 2, 3 for the first frame, 3, 1, 2 next, and then2, 3, 1. One could also use other permutation mechanisms that areadaptive to traffic conditions and traffic priorities. The difference inthe backoff delays corresponding to two consecutive tags is one timeslot. FIG. 2 illustrates the ordering of transmissions from three groupsof BSSs.

A backoff counter is associated with each backoff delay. It is decreasedaccording to the rules of TCMA using the arbitration time of ShortInterframe Space (SIFS) as described in the preceding section. That is,once the channel is idle for a time interval equal to SIFS, the backoffcounter associated with the centralized protocol of the BSS is decreasedby 1 for each slot time the channel is idle. Access attempt occurs whenthe backoff counter expires. The minimum backoff value associated withthe highest-ranking tag is 1. FIG. 3 illustrates how three interferingBSSs share the same channel for two consecutive frames. The tagsassigned in each of the two frames are (1, 2), (2, 3), and (3, 1) forthe three BSSs, respectively. The backoff delays for the three tags are1, 2, and 3 time slots.

When the channel is seized by the centralized protocol of a BSS, itengages in the polling and transmission functions for a time interval,known as the contention-free period. Once the channel has beensuccessfully accessed that way, protection by the Network AllocationVector (NAV) prevents interference from contention based trafficoriginating within that BSS. Avoidance of interference from neighboringBSS is discussed below. A maximum limit is imposed on the reservationlength in order to even out the load on the channel from different BSSsand allow sufficient channel time for contention-based traffic.

It is important to note the advantage of using deterministic backoffdelays, versus random. Assuming an efficient (i.e., compact) tag re-useplan, deterministic backoff delays increase the likelihood that a beaconwill occur precisely after an idle period of length SIFS+1=PIFS. Thiswill enable the centralized protocol to gain access to the channel, as ahigher priority class should, before contention-based traffic can accessthe channel at DIFS=PIFS+1. Using a random backoff delay instead mightimpose a longer idle period and hence, give rise to collisions withcontention-based traffic. Use of short backoff windows to avoid thisproblem would be ill advised, since that would result in collisionbetween the various BSS beacons.

Though the backoff delays are set in a deterministic manner, there areno guarantees that collisions will always be avoided. Unless theduration of the contention-free period is the same for all BSSs, thereis the possibility that interfering BSSs will attempt to access thechannel at once. In case of such a collision, the backoff procedurestarts again with the backoff delay associated with the tag assigned tothe BSS, decreased by 1, and can be repeated until expiration of theframe. At the start of a new frame, a new tag is assigned to the BSSaccording to the pre-specified sequence, and the deferral time intervalassociated with the new tag is used.

Collisions are also possible if tag assignments are imperfect(interfering BSSs are assigned the same tag). In the event of such acollision, transmission should be retried with random backoff. In orderto deal with either type of collision, resolution occurs by drawing arandom delay from a contention window size that increases with thedeterministic backoff delay associated with the tag in that frame.Though random backoff is used in this event, starting with deterministicbackoff helps reduce contention time.

In a hybrid scenario, random backoff can be combined with tagscheduling. Instead of using backoff delays linked to the rank of a tagin a frame, the contention window size from which the backoff delay isdrawn would increase with decreasing rank. The advantage of such anapproach is to relax the restrictions on re-use by allowing thepossibility that potentially interfering stations will be assigned thesame tag. The disadvantage is that the Inter-BSS Contention Period(IBCP) time needed to eliminate contention by E-DCF traffic increases.

Interference Sensing

Interference sensing is the mechanism by which the occupancy status of achannel is determined. The access point only needs to know of channelactivity in interfering BSSs. The best interference sensing mechanism isone that ensures that the channel is not used simultaneously bypotentially interfering users. This involves listening to the channel byboth the access point and stations. If the access point alone checkswhether the channel is idle, the result does not convey adequateinformation on the potential for interference at a receiving station,nor does it address the problem of interference caused to others by thetransmission, as an access point may not be able to hear transmissionsfrom its neighboring access points, yet there is potential ofinterference to stations on the boundary of neighboring BSSs. Stationsmust detect neighboring BSS beacons and forward the information to theirassociated access point. However, transmission of this information by astation would cause interference within the neighboring BSS.

In order to enable communication of channel occupancy information toneighboring access points, the invention includes the followingmechanism. When a beacon packet is transmitted, and before transmissionof any other data or polling packets, all stations hearing the beaconwill respond by sending a frame, the contention-free time response(CFTR), that will contain the duration of the contention-free periodfound in the beacon. An access point in neighboring BSSs, or stationsattempting contention-based channel access, that receive this messagefrom a station in the BSS overlapping region are thus alerted that thechannel has been seized by a BSS. Similar to a station's NetworkAllocation Vector (NAV), an Inter-Cell Network Allocation Vector, alsoreferred to herein as an inter-BSS NAV (IBNAV), is set at the accesspoint, accordingly, indicating the time the channel will be free again.Unless the IBNAV is reset, the access point will decrease its backoffvalue only after the expiration of the IBNAV, according to the backoffcountdown rules.

Alternatively, if beacons are sent at fixed time increments, receipt ofthe contention-free time response (CFTR) frame would suffice to extendthe IBNAV. The alternative would be convenient in order to obviate theneed for full decoding of the CFTR frame. It is necessary, however, thatthe frame type of CFTR be recognizable.

Contention by E-DCF traffic while various interfering BSSs attempt toinitiate their contention-free period can be lessened by adjusting thesession length used to update the NAV and IBNAV. The contention-freeperiod length is increased by a period Inter-BSS Contention Period(IBCP) during which the access points only will attempt access of thechannel using the backoff procedure, while ESTAs wait for its expirationbefore attempting transmission. This mechanism can reduce the contentionseen by the centralized protocols when employing either type of backoffdelay, random or deterministic. With deterministic backoff delays, IBCPis set equal to the longest residual backoff delay possible, which isT(slot time), where T is the number of different tags. Given reasonablere-use of the tags, the channel time devoted to the IBCP would be lesswith deterministic backoff delays, as compared to the random.

QoS Management

A QoS-capable centralized protocol will have traffic with different timedelay requirements queued in different priority buffers. Delay-sensitivetraffic will be sent first, followed by traffic with lower priority. Tagscheduling is used again, but now there are two or more backoff valuesassociated with each tag, a shorter value for the higher prioritytraffic and longer ones for lower priority. A BSS will transmit its toppriority packets first, as described before. Once the top prioritytraffic has been transmitted, there would be further delay before theBSS would attempt to transmit lower priority traffic in order to giveneighboring BSSs a chance to transmit their top priority packets. Aslong as any of the deferral time intervals for low-priority traffic islonger than the deferral time intervals for higher priority traffic ofany tag, in general all neighboring BSSs would have a chance to transmitall pending top-priority packets before any lower-priority packets aretransmitted.

FIG. 4 illustrates how three interfering BSSs, each with two types oftraffic of different priorities, share the same channel in twoconsecutive frames. As before, the tags assigned in each of the twoframes are (1, 2), (2, 3), and (3, 1) for the three BSSs, respectively.The deferral times for the top priority traffic are 1, 2, and 3 timeslots for tags 1, 2, and 3, respectively. The deferral times for thehigher priority traffic are 4, 5, and 6 time slots for tags 1, 2, and 3,respectively.

Tag Assignments

A requirement in assigning tags to BSS is that distinct tags must begiven to user entities with potential to interfere. This is not adifficult requirement to meet. In the absence of any information, adifferent tag could be assigned to each user entity. In that case,non-interfering cells will use the channel simultaneously even thoughthey have different tags. Interference sensing will enable reuse of thechannel by non-interfering BSSs that have been assigned different tags.

There are advantages, however, in reducing the number of different tags.For instance, if the interference relationships between user entitiesare known, it is advantageous to assign the same tag to non-interferingBSS, and thus have a smaller number of tags. The utilization ofbandwidth, and hence total throughput, would be greater as shorterdeferral time intervals leave more of the frame time available fortransmission. Moreover, an efficient (i.e., compact) tag re-use planwill decrease the likelihood of contention between the centralizedprotocol beacons of interfering BSSs contenting for access and E-DCFtraffic. This problem is mitigated by using the IBCP time in the IBNAV,but re-use will reduce the length of this time.

The assignment of tags to cells can be done without knowledge of thelocation of the access points and/or the stations. Tag assignment, likechannel selection can be done at the time of installation. And again,like dynamic channel selection, it can be selected by the access pointdynamically. RF planning, which processes signal-strength measurementscan establish re-use groups and thus reduce the required number of tags.FIG. 5, which includes FIGS. 5(a) and 5(b), illustrates the possiblere-use of tags. In FIG. 5(a), the access points are located at idealspots on a hexagonal grid to achieve a regular tessellating pattern. InFIG. 5(b), the access points have been placed as convenient and tags areassigned to avoid overlap. Imperfect tag assignments will lead tocollisions between the access points, but such collisions can beresolved.

To recap, arbitration times have been assigned to a centralized accessprotocol that co-exists with ESTAs accessing the channel through E-DCF.The centralized access protocol has the top priority, while E-DCF canoffer traffic classes with priority access both above and below thatprovided by legacy stations using DCF.

Table 1 illustrates the parameter specification for K+1 differentclasses according to the requirements given above. The centralizedaccess protocol is assigned the highest priority classification, andhence the shortest arbitration time. The top k−1 traffic classes for theE-DCF have priority above legacy but below the centralized accessprotocol; they achieve differentiation through the variation of thecontention window size as well as other parameters. E-DCF trafficclasses with priority above legacy have a lower bound, rLower, of thedistribution from which backoff values are drawn that is equal to 1 orgreater. Differentiation for classes with priority below legacy isachieved by increasing arbitration times; the lower bound of the randombackoff distribution can be 0.

BSSs within interfering range of one another compete for the channelthrough a deterministic backoff procedure employing tag scheduling,which rotates the backoff value for fairness among potentiallyinterfering BSS. Re-use of a tag is permitted in non-interfering BSS.Multiple queues with their own backoff values enable prioritization ofdifferent QoS traffic classes.

Contention-Free Bursts

In accordance with the invention, potential collisions between differentBSSs engaged in centralized access can be averted/resolved bydeterministic backoff delays, which avoid collisions between accesspoints, and eliminate gaps between consecutive poll/response exchangesbetween the access point and its associated stations. These are referredto as contention-free bursts (CFBs).

Deterministic Backoff Procedure for the Centralized Access Protocol

A modified backoff procedure is pursued for the beacons of thecentralized access protocols. A backoff counter is employed in the sameway as in TCMA. But while the backoff delay in TCMA is selected randomlyfrom a contention window, in the case of the centralized access protocolbeacons, the backoff value is set deterministically to a fixed valueBkoff, at the end of its contention-free session. Post-backoff is turnedon.

The backoff counter is decreased according to the rules of TCMA usingthe arbitration time AIFS=SIFS as described in the preceding section.That is, once the channel is idle for a time interval equal to SIFS, thebackoff counter associated with the centralized protocol of the BSS isdecreased by 1 for each slot time the channel is idle. Access attemptoccurs when the backoff counter expires. An HC will restart its backoffafter completing its transmission. The deterministic post-backoffprocedure is illustrated in FIG. 6.

When the channel is seized by the centralized protocol of a BSS, itengages in the polling and transmission functions for a time interval,known as the contention-free period. Once the channel has beensuccessfully accessed that way, protection by the NAV preventsinterference from contention based traffic originating in the BSS.Avoidance of interference from neighboring BSS is discussed below.

Non-Conflicting Contiguous Sequences of CFBs

As long as the value of Bkoff is greater than or equal to the maximumnumber of interfering BSS, it is possible for the contention-freeperiods of a cluster of neighboring/overlapping BSSs to repeat in thesame order without a collision between them. CFBs of different BSSs canbe made to follow one another in a contiguous sequence, thus maximizingaccess of the centralized protocol to the channel. This can be seen asfollows.

Given a sequence of successful CFBs initiated by different BSSs,subsequent CFBs will not conflict because the follower's backoff counteralways exceeds that of the leader by at least 1. If the previous CFBswere contiguous (that is, if consecutive CFBs were separated by idlegaps of length PIFS, the new CFBs will be also continuous because thefollower's backoff delay exceeds that of the leader by exactly 1.Channel access attempts by E-DCF stations require an idle gap of lengthequal to DIFS or greater. FIG. 7 shows the relationships of repeatingsequences of CFBs.

In order to maintain contiguity, an HC that does not have any traffic totransmit when its backoff expires, it will transmit a short packet—a“peg”—and then engage in post-backoff. This way no gaps of length DIFS+1are left idle until all HCs have completed one CFB per cycle, andrestarted the backoff countdown procedure. E-DCF stations are thusprevented from seizing the channel until each BSS completes at least oneCFB per cycle. FIG. 8 illustrates the role of pegging in a sequence ofCFBs by three overlapping access points.

Finally it is shown how such a contiguous sequence can constructed byanalyzing how a new access point initiates its first CFB. Every time anew access point is installed, it must find its position in therepeating sequence of CFBs. The new access point listens to the channelfor the desired cycle, trying to recognize the sequence. It listens foran “idle” PIFS following a busy channel. When that occurs, or aftercounting Bkoff time slots, whichever comes first, the new access pointstarts looking for the first idle longer than PIFS, which signifies theend of the sequence of CFBs. As long as the Bkoff is greater than thenumber of interfering BSS, there will always be such an idle period. Theaccess point sets its post-backoff delay so that it transmits alwaysright at the end of the CFB sequence. That is, if at time t, anidle>PIFS has been detected, the access point's backoff at time t isBkoff−x(t), where x(t) is the number of idle time slots after PIFS. FIG.9 illustrates this start-up procedure for a new access point, HC2, givenan existing access point, HC1.

Interference Sensing

Interference sensing is the mechanism by which the occupancy status of achannel is determined. The access point only needs to know of channelactivity in interfering BSSs. The best interference sensing mechanism isone that ensures that the channel is not used simultaneously bypotentially interfering users. This involves listening to the channel byboth the access point and stations. If the access point alone checkswhether the channel is idle, the result does not convey adequateinformation on the potential for interference at a receiving station,nor does it address the problem of interference caused to others by thetransmission, as an access point may not be able to hear transmissionsfrom its neighboring access points, yet there is potential ofinterference to stations on the boundary of neighboring BSS. Stationsmust detect neighboring BSS beacons and forward the information to theirassociated access point. However, transmission of this information by astation would cause interference within the neighboring BSS.

In order to enable communication of channel occupancy information toneighboring access points, the following mechanism is proposed. When abeacon packet is transmitted, and before transmission of any other dataor polling packets, all stations not associated with the access pointthat hear the beacon will respond by sending a frame, thecontention-free time response (CFTR), that will contain the duration ofthe contention-free period found in the beacon. An associated stationwill transmit the remaining duration of the contention-free period whenpolled. An access point in neighboring BSSs, or stations attemptingcontention-based channel access, that receive this message from astation in the BSS overlapping region are thus be alerted that thechannel has been seized by a BSS. Similar to a station's NAV, aninter-BSS NAV (IBNAV) will be set at the access point accordinglyindicating the time the channel will be free again. Unless the IBNAV isreset, the access point will decrease its backoff value only after theexpiration of the IBNAV, according to the backoff countdown rules.

Alternatively, if beacons are sent at fixed time increments, receipt ofthe CFTR frame would suffice to extend the IBNAV. The alternative wouldbe convenient in order to obviate the need for full decoding of the CFTRframe. It is necessary, however, that the frame type of CFTR berecognizable.

Contention by E-DCF traffic while various interfering BSSs attempt toinitiate their contention-free period can be lessened by adjusting thesession length used to update the NAV and IBNAV. The contention-freeperiod length is increased by a period IBCP (inter-BSS contentionperiod) during which the access points only will attempt access of thechannel using the backoff procedure, while ESTAs wait for its expirationbefore attempting transmission. This mechanism can reduce the contentionseen by the centralized protocols when employing either type of backoffdelay—random or deterministic.

QoS Management

A QoS-capable centralized protocol will have traffic with different timedelay requirements queued in different priority buffers. Delay-sensitivetraffic will be sent first, followed by traffic with lower priority. ABSS will schedule transmissions from separate queues so that the QoSrequirements are met. It will transmit its top priority packets first,as described before. Once the top priority traffic has been transmitted,the BSS would attempt to transmit lower priority traffic in the CFBsallotted.

Three parameters are employed to help manage QoS. The deterministicbackoff delay, Bkoff, and the maximum length of a CFB and of a DCFtransmission. Since these parameters determine the relative allocationof the channel time between the centralized and distributed protocols,they can be adjusted to reflect the distribution of the traffic loadbetween the two protocols. It must be kept in mind, however, that thesame value of Bkoff should be used by all interfering BSSs.

QoS Guarantees

To enable high priority traffic to be delivered within guaranteedlatency limits, a variation of the above method is described. CFBs of anaccess point are separated into two types, or tiers. The first containstime sensitive data and is sent when the period TXdt expires. The secondtier contains time non-sensitive traffic and is sent when the backoffcounter expires as a result of the countdown procedure. When allneighboring BSS have a chance to transmit their time sensitive traffic,the channel is available for additional transmissions before needing totransmit time-sensitive traffic again. Lower priority contention-freedata can be then transmitted, using a backoff-based procedure.

Tier II CFBs can be initiated in various methods. Two will be describedhere. They are: (1) random post-backoff, and (2) deterministicpost-backoff. Both methods use the same AIFS used for top-priority EDCFtransmissions, in order to avoid conflict with Tier I CFBs (i.e. anAIFS=PIFS). Conflict with top priority EDCF transmissions can bemitigated in case (1) or prevented in case (2) through the use of theIBNAV with an IBCP.

Random post-backoff assigns an access point a backoff drawn from aprespecified contention window. A short contention window would lead toconflicts between Tier II CFBs. A long contention window reduces theconflict between interfering BSS attempting to access the channel atonce. Long backoff values would reduce the fraction of the time thechannel carries CFBs. Furthermore, the gaps created by multipleconsecutive idle slots make room for DCF transmissions, reducing furtherthe channel time available to CFBs. A long IBCP value would alleviatesome of the conflict with DCF transmissions.

Deterministic post-backoff eliminates the problems present with randompost-backoff. Conflicts with top priority EDCF transmissions can beprevented with an IBCP of 1. Moreover, as explained above, the Tier IICFBs generated by this method, do not conflict with one another and formcontiguous repeating sequences.

Non-Conflicting Contiguous Sequences of Tier I CFBs

Periodic transmission is achieved by maintaining a timer which is resetat the desired period TXdt as soon as the timer expires. A CFB isinitiated upon expiration of the timer. As long as Tier Icontention-free periods are all made the same size (by adding timenon-critical traffic), which is not less than the maximum DCFtransmission or Tier II CFB length, it is possible for thecontention-free periods of a cluster of neighboring/overlapping BSSs torepeat in the same order without a collision between them. CFBs ofdifferent BSSs can be made to follow one another in a contiguoussequence, thus maximizing access of the centralized protocol to thechannel. This can be seen as follows.

Given a sequence of successful CFBs initiated by different BSSs,subsequent CFBs will not conflict because their timers will expire atleast TICFBLength apart. If the leading access point's timer expireswhile the channel is busy, it will be able to start a new CFB before thefollower HC because DCF transmissions are of equal or shorter length,and Type II CFBs have equal or shorter length.

If the previous CFBs were contiguous (that is, if consecutive CFBs wereseparated by idle gaps of length PIFS), the new CFBs will be alsocontinuous because the follower's timer will expire on or before thecompletion of the leader's CFB because their CFBs have the same length.Channel access attempts by E-DCF stations or Tier II CFBs require anidle gap of length equal to DIFS or greater, and hence they cannot beinterjected. FIG. 10 shows the relationship of repeating sequences ofTier I CFBs.

Finally it is shown how such a contiguous sequence can constructed byanalyzing how a new access point initiates its first Tier I CFB. Everytime a new access point is installed, it must find its position in therepeating sequence of CFBs. The new access point listens to the channelfor the desired cycle, trying to recognize the sequence. It listens foran “idle” PIFS following a busy channel. When that occurs, or after aperiod TXdt, whichever comes first, the new access point starts lookingfor the first idle longer than PIFS, which signifies the end of thesequence of Tier I CFBs. As long as the TXdt is greater than the numberof interfering BSS times the duration of a Tier I CFB, TICFBLength,there will always be such an idle period. The access point sets itstimer so that it transmits always right at the end of the CFB sequence.That is, if at time t, an idle of length X(t)>PIFS has been detected,the access point's timer at time t is TXdt−X(t)+PIFS. FIG. 11illustrates this start-up procedure for a new access point, HC2, givenan existing access point, HC1.

Possibility of Collisions

Though the backoff delays are set in a deterministic manner, there areno guarantees that collisions will always be avoided. Unless all accesspoints sense the start and end of CFBs at the same time, there is thepossibility that interfering BSSs will attempt to access the channel atonce. This situation arises when there is significant distance betweenaccess points, but not sufficient to eliminate interference betweenthem. Such a situation can be alleviated through the assignment fordifferent channels.

Arbitration times are assigned to a centralized access protocol thatco-exists with ESTAs accessing the channel through E-DCF. Thecentralized access protocol has the top priority, while E-DCF can offertraffic classes with priority access both above and below that providedby legacy stations using DCF.

Table 1 illustrates the parameter specification for K+1 differentclasses according to the requirements given above. The centralizedaccess protocol is assigned the highest priority classification, andhence the shortest arbitration time. The top k−1 traffic classes for theE-DCF have priority above legacy but below the centralized accessprotocol; they achieve differentiation through the variation of thecontention window size as well as other parameters. E-DCF trafficclasses with priority above legacy have a lower bound, rLower, of thedistribution from which backoff values are drawn that is equal to 1 orgreater. Differentiation for classes with priority below legacy isachieved by increasing arbitration times; the lower bound of the randombackoff distribution can be 0. TABLE 1 TCMA Priority Class DescriptionPriority Class Description Arbitration time rLower 0 Centralized accessprotocol SIFS >=1 CFBs 1 to k − 1 E-DCF Traffic with priority PIFS =SIFS + 1 >=1 above Legacy or Centralized (slot time) access protocolTier II CFBs k E-DCF Legacy-equivalent DIFS = SIFS + 2 0 trafficpriority (slot time) n = k + 1 E-DCF Traffic priority below >DIFS =SIFS + 0 to K Legacy (2 + n − k) (slot time)

BSSs within short interfering range of one another can compete for andshare the channel through the use of a deterministic backoff procedureemploying post-backoff. Contiguous repeating sequences ofcontention-free periods provide the centralized protocol efficientaccess to the channel which is shared by E-DCF transmissions. Therelative channel time allotted to the two protocols can be adjusted bytuning parameters of the protocol. Scheduling of traffic queued inmultiple queues at the access point can meet QoS requirements. Morestringent latency requirements can be met with a two-tiered method,which employs both a timer and post-backoff to initiate CFBs.

CFB contiguity is preserved when using deterministic post-backoff or ifCFBs of constant length are used whenever transmission is caused by theexpiration of the TXdt timer—the Tier I approach. Contiguity is notnecessarily preserved, however, if the CFBs have variable length whenthe Tier I approach is used. Any gaps that would arise in this casewould allow contention-based transmissions to be interjected, thusrisking delays and possible collisions between HCs.

Because of the fixed CFB length requirement, whereas the Tier I approachdelivers regularly-spaced CFBs, using it alone, without a Tier IIprotocol, results in inefficient utilization of the channel. The samefixed bandwidth allocation to each BSS gives rise to situations wherechannel time allocated for a CFB to one BSS may be left idle whileanother BSS is overloaded. The Tier II protocols provide for dynamicbandwidth allocation among BSSs.

Various illustrative examples of the invention have been described indetail. In addition, however, many modifications and changes can be madeto these examples without departing from the nature and spirit of theinvention.

1. A method for enabling a plurality of overlapped wireless LAN cells to have contention-free access to a medium, each cell including a respective plurality of member stations, comprising: assigning a first inter-cell contention-free period value to a first access point station in a first cell, associated with an accessing order in the medium for member stations in said first cell and a second cell of the plurality of cells during a transmission frame; transmitting by the first access point station in the first cell, a beacon packet containing the first inter-cell contention-free period value; and receiving the beacon packet at an overlapped station occupying the first cell and the second cell, and forwarding the first inter-cell contention-free period value to member stations in the second cell, to delay transmissions by member stations in the second cell until after said first inter-cell contention free period.
 2. The method of claim 1, further comprising: assigning a second inter-cell contention-free period value to a second access point station in the second cell, associated with said accessing order; transmitting by the second access point station in the second cell, a second beacon packet containing the second inter-cell contention-free period value; and receiving the second beacon packet at an overlapped station occupying said first cell and said second cell, and forwarding the second inter-cell contention-free period value to member stations in the first cell to delay transmissions by member stations in the first cell until after said second inter-cell contention-free period.
 3. The method of claim 2, further comprising: receiving the first inter-cell contention-free period value and the second inter-cell contention-free period value at a third access point station in a third cell overlapped with said first cell and said second cell; selecting a backoff time for said third access point station following the second inter-cell contention-free period, to begin transmission by the third access point station in the third cell of a third beacon packet containing a third inter-cell contention-free period value; and receiving the third beacon packet at an overlapped station occupying said first cell and an overlapped station occupying said second cell, and forwarding the third inter-cell contention-free period value to member stations in the first cell and the second cell to delay transmissions by member stations in the first cell and the second cell until after said third inter-cell contention-free period.
 4. The method of claim 2, further comprising: transmitting in the first beacon packet to member stations in the first cell, an intra-cell contention-free period value, during which they will delay accessing the medium.
 5. The method of claim 2, further comprising: transmitting a peg packet by the second access point station in the second cell to deter member stations in said overlapped first cell from contending the medium.
 6. A method for enabling a plurality of overlapped wireless LAN cells to have contention-free access to a medium, each cell including a respective plurality of member stations, comprising: assigning a first inter-cell contention-free period value to a first access point station in a first cell, associated with an accessing order in the medium for member stations in said first cell and a second cell of the plurality of cells during a transmission frame; transmitting by the first access point station in the first cell, a beacon packet containing the first inter-cell contention-free period value; assigning a second inter-cell contention-free period value to a second access point station in the second cell, associated with said accessing order; and transmitting by the second access point station in the second cell, a second beacon packet containing the second inter-cell contention-free period value.
 7. The method of claim 6, further comprising: receiving the first inter-cell contention-free period value and the second inter-cell contention-free period value at a third access point station in a third cell overlapped with said first cell and said second cell; and selecting a backoff time for said third access point station following the second inter-cell contention free period, to begin transmission by the third access point station in the third cell of a third beacon packet containing a third inter-cell contention-free period value.
 8. The method of claim 7, further comprising: receiving the third inter-cell contention-free period value at member stations in the first cell and the second cell to delay transmissions by member stations in the first cell and the second cell until after said third inter-cell contention-free period.
 9. The method of claim 6, further comprising: transmitting in the first beacon packet to member stations in the first cell, an intra-cell contention-free period value, during which they will delay accessing the medium.
 10. The method of claim 7, further comprising: transmitting a peg packet by the third access point station in the third cell to deter member stations in said overlapped first cell from contending for the medium.
 11. A system for enabling a plurality of overlapped wireless LAN cells to have contention-free access to a medium, each cell including a respective plurality of member stations, comprising: means for assigning a first inter-cell contention-free period value to a first access point station in a first cell, associated with an accessing order in the medium for member stations in said first cell and a second cell of the plurality of cells during a transmission frame; means for transmitting by the first access point station in the first cell, a beacon packet containing the first inter-cell contention-free period value; and means for receiving the beacon packet at an overlapped station occupying the first cell and the second cell, and forwarding the first inter-cell contention-free period value to member stations in the second cell, to delay transmissions by member stations in the second cell until after said first inter-cell contention free period.
 12. The system of claim 11, further comprising: means for assigning a second inter-cell contention-free period value to a second access point station in the second cell, associated with said accessing order; means for transmitting by the second access point station in the second cell, a second beacon packet containing the second inter-cell contention-free period value; and means for receiving the second beacon packet at an overlapped station occupying said first cell and said second cell, and forwarding the second inter-cell contention-free period value to member stations in the first cell to delay transmissions by member stations in the first cell until after said second inter-cell contention-free period.
 13. The system of claim 12, further comprising: means for receiving the first inter-cell contention-free period value and the second inter-cell contention-free period value at a third access point station in a third cell overlapped with said first cell and said second cell; selecting a backoff time for said third access point station following the second inter-cell contention-free period, to begin transmission by the third access point station in the third cell of a third beacon packet containing a third inter-cell contention-free period value; and receiving the third beacon packet at an overlapped station occupying said first cell and an overlapped station occupying said second cell, and forwarding the third inter-cell contention-free period value to member stations in the first cell and the second cell to delay transmissions by member stations in the first cell and the second cell until after said third inter-cell contention-free period.
 14. The system of claim 12, further comprising: means for transmitting in the first beacon packet to member stations in the first cell, an intra-cell contention-free period value, during which they will delay accessing the medium.
 15. The system of claim 12, further comprising: means for transmitting a peg packet by the second access point station in the second cell to deter member stations in said overlapped first cell from contending the medium. 